Holds a Locale::Maketext (or other duck_type class with a 'maketext' method) language handle.
The language handle is used to localize the error messages in the field's 'add_error' method.
It's also used in various places in rendering to localize labels and button values,
etc.

The builder for this attribute gets the Locale::Maketext language handle from the environment variable $ENV{LANGUAGE_HANDLE}:

$ENV{LANGUAGE_HANDLE} = 'en_en';

...or creates a default language handler using HTML::FormHandler::I18N. (Note that earlier versions required an actual object reference in ENV, which is a bad practice and no longer supported.) You can pass in an existing Locale::MakeText subclass instance or create one in a builder.